In mass spectrometers it is common to employ ion optical components which have an alternating current (“AC”) voltage waveform, e.g. a radiofrequency (“RF”) voltage waveform, applied thereto, e.g. for the purpose of containing charged particles. Examples of ion optical components include multipole devices (such as quadrupole, hexapoles, octapoles etc), 3D ion traps, stacked ring ion guides, mass filters, ion funnels, linear ions traps, ion guides. Other examples exist. Frequently, several ion optical components might be employed in a device, such as a mass spectrometer, in combination, where they might serve different purposes. For example, an ion funnel might be employed to capture ions at the inlet of a mass spectrometer, before the ions are transferred into a hexapole and then onwards into a mass filter before being detected.
Often, ions are transferred from one ion optical component to another (or within one ion optical component) by using DC offset voltages, e.g. to create a DC gradient. For example, a DC gradient going from a more positive potential to a more negative potential would tend to move positive ions from the region of more positive potential to the region of more negative potential. Negative ions would experience the reverse force and would tend to be moved from the region of more negative potential to the region of more positive potential. An example of such a DC offset scheme is shown in FIG. 1. Here, a higher DC offset voltage is applied to the first ion optical element 1. The DC offset voltage profile 11 varies in magnitude along the length. A DC offset voltage profile such as that shown in FIG. 1 might be used to transfer positively charged ions into the fourth ion optical component 7 and trap them there (assuming adequate attention is paid to cooling of the ions to reduce their translational energy).
In some cases, several ion optical devices might have the same AC voltage waveform applied, but might be required to have different DC offset voltages. One example of such a situation might be a segmented ion guide device, where several segments each have the same applied AC voltage waveform, but have different DC offset potentials.
The inventors have observed that when changing a DC offset voltage produced at a DC power supply from an initial DC offset voltage to a target DC offset voltage, it can take some time for a corresponding change in DC offset voltage to take place at a component to which the DC offset voltage is applied. The inventors believe it may be desirable for the change in DC offset voltage at the component to take place more quickly (as might be useful in cases where time is critical) and/or to take place at a preferred time (as might be useful where it is desired for changes in DC offset voltages at multiple components to take place in the same time window).
The present invention has been devised in light of the above considerations.
By way of background:                Paul and Steinwedel in 1953 (Z. Naturforsch, 1953, 8a, 448 describes a quadrupole mass analyser.        Horowitz and Hill, 1989, “The Art of Electronics”, Second edition, pages 23-24 describes the physical response of an RC network.        U.S. Pat. No. 8,759,759B2 discloses a linear ion trap mass analyzer comprised by multiple columnar electrodes. FIG. 5 of this document provides a schematic of an RC coupling network. This figure is referenced in paragraph [0047] where the circuit of FIG. 5 is described as being “used to superimpose [a] high frequency voltage component and [a] field-adjustable DC voltage component”        U.S. Pat. No. 8,030,613B2 discloses a radio frequency (RF) power supply in a mass spectrometer. FIG. 3 of this document shows a schematic of a circuit used to apply a DC offset by way of a centre tapped transformer.        